The bind mounts.
Since Linux 2.4.0 it is possible to remount part of the file hierarchy somewhere else. The call is
mount --bind olddir newdir
or shortoption
mount -B olddir newdir
or fstab entry is:
/olddir /newdir none bind

After this call the same contents is accessible in two places. One can also remount a single file (on a single
file).

This call attaches only (part of) a single filesystem, not possible submounts. The entire file hierarchy including
submounts is attached a second place using
mount --rbind olddir newdir
or shortoption
mount -R olddir newdir

Note that the filesystem mount options will remain the same as those on the original mount point, and cannot be
changed by passing the -o option along with --bind/--rbind. The mount options can be changed by a separate remount
command, for example: